Influence of large South American rivers of the Plata Basin on distributional patterns of tropical snakes: a panbiogeographical analysis

Abstract

Aim: The main drainages of the Plata Basin ? the Parana´ , Paraguay and Uruguay rivers ? begin in tropical latitudes and run in a north?south direction into subtropical?temperate latitudes. Consequently, the biota of these rivers has tropical elements that contrast with temperate biomes through which the rivers run. We apply a panbiogeographical approach, to test whether the large rivers of the Plata Basin have a differential influence on distributional patterns of tropical snakes in subtropical and temperate latitudes of South America. Location: Subtropical and temperate sections of the major Plata Basin rivers, South America. Methods: We compared the individual tracks of 94 snake taxa. The track analysis consisted of: (1) plotting the localities of each taxon on maps, (2) connecting the localities of each taxon using a minimal geographical proximity determinant of the ?individual tracks?, and (3) superimposing the individual tracks to determine generalized tracks. To detect tropical snakes that reach higher latitudes through the rivers we used the preferential direction of distribution concept. For each taxon we measured the angular deviations between the line of its individual track and the course of the rivers in a 100 · 100 km scaled grid. Average angular values < 45 indicated a positive association with the rivers. Results: Thirty-five of 94 taxa showed distributions associated with the major rivers of the Plata Basin, including fauna from distinct biogeographical lineages, supported by the occurrence of five generalized tracks as follows: (1) the Paraguay?Middle Parana´, (2) the Paraguay?Parana´ fluvial axis, Upper Parana´ and Middle Parana´ to Upper Delta, (3) the Lower Paraguay, Parana´ and Uruguay rivers, excluding the sectors High Parana´ and High Uruguay, (4) the Uruguay River and Upper Parana´, and (5) the High Parana´ . The Atlantic species occurred with significantly higher frequency in the Uruguay River and High Parana´ river sections, the Amazon species were found with significantly higher frequency in the Paraguay and Middle Parana´ sections, and the species with a Pantanal distribution were found in all sections. Main: conclusions The observed distributional patterns may be explained by the interaction of ecological, geographical and historical factors. Previous authors have developed ecological (hydrological or environmental similarity) or dispersalist (effect of rivers as migration routes) explanations. The coincidence between generalized tracks and past geomorphological events that caused displacements and changed relationships between the Paraguay, Parana´ and Uruguay river sections supports hypotheses involving the strong influence of historical factors in the present configuration of tropical snake distribution in temperate latitudes.